1. Field of Invention
The invention relates to capacitive touch panels, and in particular to a touch detection method and circuit for capacitive touch panels.
2. Description of Related Arts
FIG. 1 is a general flow chart of a capacitive touch detection method, in which 110 is an equivalent capacitance touch panel. In order to reduce the load of a microprocessor, the scanning of a capacitive touch panel is typically performed row by row or column by column, that is, one row or column is scanned at the same time. In order to represent a touch-caused capacitance change, the capacitance of a scanned row or column is linearly converted to a variable distinguishable to an analog circuit, such as time or frequency, then the variable is subjected to an analog-to-digital conversion, and the result of the conversion is sent to a microprocessor to determine whether there exists a touch.
At present, there are two methods for detecting a touch on a capacitive touch panel. In the first method represented by SiliconLab, a RC is taken as a time constant of an oscillator and different capacitors C are corresponding to different periods or frequencies. The specific realization circuit of this kind of method is illustrated in FIG. 2. In the second method represented by Cypress, the capacitance of a touch capacitor is first converted to the resistance of a resistor by means of the equivalence of a switched capacitor to a resistor, and then the resistor is used to charge or discharge a constant capacitor to detect the change in a corresponding voltage. The specific realization circuit of this kind of method is illustrated in FIG. 4.
In FIG. 2, Ct is a capacitor to be detected. When a comparator Vout outputs a high voltage Vdd, the voltage at the positive terminal of the comparator is equal to 2/3 Vdd, and Ct is charged by Vdd via an R4. The comparator Vout outputs a zero voltage when Ct is charged to 2/3 Vdd, then the voltage at the positive terminal of the comparator is equal to 1/3 Vdd, and Ct is discharged to the ground via R4. When the voltage of Ct is discharged to 1/3 Vdd, the Vout outputs the Vdd to repeat the processing above. FIG. 3 shows voltage waveforms of the Vout and the Ct.
In FIG. 4, Ct is a capacitor to be detected, with a resistance Rt of 1/f*Ct (f is a switching frequency), Cm is a fixed capacitor, and Iref is a constant charging current. The greater the capacitance of Ct is, the smaller Rt is, and the lower the charging voltage of Cm is at the same time. Thus, the capacitance value of Ct is converted to a corresponding voltage.
Only a single row or column is scanned at the same time by using said two touch detection methods of Silicon Lab Corporate and Cypress Corporate, which determine the change of a touch capacitance by converting the change of the touch capacitance to that of a frequency, period or voltage; however, the interference on a panel is different at a different period of time, therefore, the result of the detection may be wrong or a detection coordinate may drift with respect to a touch point, leading to a poor interference resistance and an error determination.